The irony here is that XML Schema is actually not powerful enough to
express the *more* restrictive alternative that Leigh expressed in RNG.
Separately, it is the normative spec prose which also expresses the
content models of all elements, and the content model for select and
select1 currently does not allow UI common elements before List UI common.
Although the XML schema is separately non-normative, it would be
important to be able to express a reasonable approximation of the prose in
XML Schema just to enable machine validation via an important tool in the
XML technology stack.
So, drilling into this a bit more, is it really the case that XML schema
cannot allow the more permissive child element sequence you desire?
I think XML schema can easily provide an appropriately permissive
definition for List.UI.Common, so I'd challenge you to prove me wrong, and
let's not put the axe to XML schema without such a proof.
:-)
John
From:
Erik Bruchez <ebruchez@orbeon.com>
To:
Forms WG <public-forms@w3.org>
Date:
09/21/2010 02:12 PM
Subject:
Re: XForms 1.1 erratum for select and select1 and ordering of
help/hint/alert/action
John,
I don't want to create more work than needed for such a small change.
It's just a shame that we have to specify our grammar in an unsatisfactory
way just because XML Schema is too weak to accomodate something that
simple. In other words XML Schema shows once again to be the wrong tool to
describe or supplement our grammar.
And I say unsatisfactory because the trend on the web these days is not to
constrain more, but to constrain less, and certainly not to constrain when
unnecessary. Here the restriction seems unnecessary both for implementors
and form authors - it just seems necessary for the schema authors ;)
And then there is what we say in the spec and schema, and what
implementations and form authors will actually do. My guess is that
implementations typically do not enforce these constraints in general,
simply because there is no need to. If that's the case, our schema is kind
of irrelevant for this particular purpose.
This said I don't have a good solution either (except for dropping XML
Schema altogether but that's probably another topic).
-Erik
On Mon, Sep 20, 2010 at 3:27 PM, John Boyer <boyerj@ca.ibm.com> wrote:
Well, allowing full and arbitrary mixing of UI common and list UI common
elements is what I thought Leigh was asking me to do.
It's one thing to say to do it "whatever the impact on the XForms schema
might be" but it's another to be the person who has to do the surgery on
both the XML schema and RNG schema, and then *sell* that as a minor
change.
In this regard, I like the working group approach better because it is a
minimal schema change that has maximal effect in terms of what authors are
actually likely to do. They probably are likely to group the UI common
elements together. The real issue we're trying to fix is that they are
likely to group them together right after the label element, for
consistency with other UI controls. The schema as-is is certainly too
restrictive. The working group decided to fix it by allowing UI.common
before List UI common as well as after. That's been done. A more
powerful solution *could* be achieved as long as someone is willing to
spend time on it, but someone != me because because it does not give much
practical benefit over the solution we already chose. But if this topic
is a high priority for anyone else, I certainly don't object to them
taking it on.
Anyway, Leigh did an alternative change in RNG, before the WG decision.
It is slightly more restrictive, and now he's asking whether we should go
with that or go with the solution encoded in the XML schema.
I liked the working group's solution because it was the most certain to
work in XML schema, but I also like Leigh's solution because it is the
minimum change of validating content that still solves the problem of
allowing author consistency across UI elements.
However, something felt a little fishy about it, so I asked a co-worker at
my office to try out the XML schema fragment that seemed to match Leigh's
RNG expression. Unfortunately, it does not work and it appears that XML
schema does not allow this type of construct, at least not without more
effort (and please see above for my view on how much effort I think should
be spent on this).
The technical error we get is "Unique Particle Attribution" error. This
happens because XML schema expects to be able to unambiguously determine
what is going on with only local content particles (a constant-sized
lookahead). When you have
<select 1 ...>
<label>...
<itemset ...> ...
</select1>
It would validate against both (UI.Common* List.UI.Common+) and
(List.UI.Common+ UI.Common*).
However, it looks like XML schema processors want to choose which one of
these to use based on at most the first element after label, the <itemset>
in this case. But because we have UI.Common* at the front of the first
rule, both it and the second rule are applicable, which is ambiguous.
The problem worsens, I think, with
<select 1 ...>
<label>...
<itemset ...> ...
<hint>...
</select1>
because clearly only the second rule applies, but apparently XML schema
only looks as far as the itemset and gives up before knowing that one of
the rules will fail and the other will succeed.
It appears that RNG keeps going and allows the content if one or more of
the choices will validate the content.
Long story short, the most expedient solution to our problem is to keep
the erratum as-is and change the RNG schema.
Cheers,
John
From:
Erik Bruchez <ebruchez@orbeon.com>
To:
public-forms@w3.org
Date:
09/20/2010 11:34 AM
Subject:
Re: XForms 1.1 erratum for select and select1 and ordering of
help/hint/alert/action
It seems to be an unnecessary constraint for the form author to
mandate that UI Common elements be all grouped together. So my vote
goes for the more lax solution, whatever the impact on the XForms
schema might be.
-Erik
On Mon, Sep 20, 2010 at 10:35 AM, Leigh L Klotz Jr
<leigh.klotz@xerox.com> wrote:
> John and I implemented the select and select1 content model erratum
> differently.
>
> At the time, we polled implementors and asked if any cared about whether
the
> help/hint/alert/action had any ordering constraints against item and
> itemset, and the answer was no.
>
> Unfortunately, we're in a state at the moment with the proposed erratum
in
> the wiki [errata] and the RNG schema are at odds.
>
> So the question is: Must UI Common content all be together, or can be be
> split with some before the List and some after?
>
> Since we asked implementors before what they accepted, I'd like to ask
again
> if anyone cares so we can fine-tune the erratum and the schema.
>
> Details:
> Here's what the XForms 1.1 RNC says:
>
> xforms.select1.content =
> xforms.label, ((xforms.UI.Common.content,
xforms.List.UI.Common.content) |
> (xforms.List.UI.Common.content, xforms.UI.Common.content))
>
> The proposed erratum says this:
> label, (UI Common)*, (List UI Common)+, (UI Common)*
>
> To be consistent with the proposed erratum, we'd change the RNC to this:
>
> xforms.select1.content =
> xforms.label, xforms.UI.Common.content, xforms.List.UI.Common.content,
> xforms.UI.Common.content
>
> Or, we would change the proposed erratum to this:
> label, ((UI Common)*, (List UI Common)+) | ((List UI Common)+, (UI
> Common)*)
>
>
> [errata]
> http://www.w3.org/MarkUp/Forms/wiki/XForms_1.1_First_Edition_Errata
>
> Leigh.
>
>